"We buy regular because it's cheaper," said the Ma'ili resident. "Gas is gas, no matter what they say about it."

But for thousands of Hawai'i drivers, regular just isn't good enough. Statewide, about 21 percent of drivers buy premium-grade, compared with about 13 percent nationally. Meanwhile, only about 6 percent of cars sold in the United States require the more expensive gasoline, according to AAA travel club.

Why more consumers in Hawai'i tend to fill up with premium is unclear, but in general, the perception is that the more-expensive gasoline is better, said Jeffrey Spring, a spokesman for AAA travel club. In fact, it could be worse, he said.

"Some people think they get better performance or gas mileage, but it's just a myth," Spring said. "I'm sure it's been helped by certain marketing efforts by the oil companies."

The major difference between gasoline grades is the level of octane. Higher octane levels typically are required in high-compression engines found in some sports or luxury cars to prevent engine knock — a potentially damaging condition when the fuel-air mixture in an engine's cylinders ignites prematurely.

But there's no independent research that shows engines designed for regular gasoline last longer or run better on premium.

Apart from octane ratings, the Environmental Protection Agency requires that all grades of gasoline contain engine-cleaning detergents to protect against the harmful build-up of engine deposits. However, ChevronTexaco Corp. said its premium not only has a higher octane rating, but additional Techron — an additive designed to cut emissions and engine deposits.

"We do want people to know that when you do buy up in grade, you are getting additional stuff, that there is more value," said ChevronTexaco spokesman Albert Chee.

Whether you get a cleaner engine from additives has been debated. As a rule, higher-octane gasoline doesn't outperform other grades when it comes to preventing or removing engine deposits, said Jeffrey Klurfeld, regional director for the FTC's Western region.

Michael Frei with The Auto Doctor said a higher-grade gasoline won't prevent engine deposits or noticeably increase performance except in limited circumstances.

The one significant difference between regular and premium gasoline is price. Premium can cost 15 to 20 cents a gallon more than regular.

"With today's prices, people really need to be smart about the fuel they use," Frei said. "I'm a technician and I feel (premium) is a waste of money."

The FTC, EPA, AAA and the American Petroleum Institute suggest drivers use the grade of gasoline recommended by a car's manufacturer.

"If they don't need (premium), they're paying for something they don't need," Klurfeld said. "If you want to spend money you don't need to, buy premium."